Low-temperature heterogeneous catalytic oxidation of
CO and VOCs
Petya Mitkova Konova
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences,
Acad. G. Bonchev St., Bl. 11, 1113 Sofia, Bulgaria
E-mail: [email protected]
Efficient oxidation of CO and volatile organic compounds (VOCs) to nontoxic products
as СО2 and Н2О at very low temperatures is a considerable problem, which can be solved
using two approaches: (i) by oxidation with ozone as oxidizing agent stronger than oxygen, or
(ii) by finding catalysts, very active at low temperature. Therefore the main goals of our work
are: (i) synthesis and characterization of new types catalysts on the base of transition metal
oxides supported on different porous materials and investigation of their properties in respect
to ozone decomposition and oxidation of CO and VOCs with ozone; (ii) investigation of the
properties of highly-active catalysts on the base of nanosized gold supported on transition
metal oxides in respect to low-temperature CO oxidation with oxygen.
Three different types of catalysts are subject of our investigation. The first group
includes catalysts containing alumina supported unstable oxides of nickel and cobalt in higher
(+3 and +4) oxidation states, which possess strong oxidation activity. The second group is
more diverse and contains catalysts on the base of silver supported microporous and
mesoporous materials. The third group consists of the catalysts Au/TiO2 and Au/ZrO2, which
contain highly active nanosized gold.
Alumina-supported nickel and cobalt oxide systems with overstoichiometric oxygen
(NiOx/Al2O3 and CoOx/Al2 O3) were investigated in respect to heterogeneous catalytic
decomposition of ozone, complete oxidation of volatile organic compounds (VOCs) and
oxidation of CO in presence of different oxidizing agents (ozone or oxygen). The experiments
were performed in the temperature range of -50oC to 250oC in an isothermal plug–flow
reactor. The catalysts were prepared by a deposition oxidation-precipitation method and were
characterized by chemical analysis, XPS, XRD, IR techniques, magnetic and adsorption
measurements.
1
The catalysts NiOx/Al2O3 and CoOx/Al2O3 show a very high activity in both reactions of
ozone decomposition and oxidation with ozone. Ozone decomposition proceeds with high
conversion even at temperatures below 0°C and reaches 100% at room temperature. Both of
the catalysts remain active with the time and do not show any deactivation. Oxidation with
ozone instead of oxygen leads to a higher catalytic activity at lower temperatures by change of
the oxidation reaction mechanism. The NiOx/Al2 O3 catalyst shows a higher activity towards
reaction of ozone decomposition, whereas CoOx/Al2O3 catalyst has a better activity in respect
to complete oxidation of VOCs and oxidation of CO.
Two main reasons for the high catalytic activity of NiOx/Al2O3 and CoOx/Al2 O3
catalysts are found: (i) the high content of active and mobile oxygen obtained during the
synthesis on the catalyst surface; (ii) the catalytic active complexes of Ni4+ and Co4+, which
are formed during the reaction of ozone decomposition and are able to oxidize VOCs and CO
at low temperature.
The second object of our investigation are silver supported mesoporous and
microporous catalysts, because silver oxides are formed easily in ozone-containing
environment and they are unstable and decompose rapidly, releasing highly active oxygen.
Silver modified (5 and 2 wt%) mesoporous molecular sieves (H-MCM-41, with Si/Al ratio
20, 40 and 50), zeolite catalysts (H-Beta, with Si/Al ratio 11 and 25; H-ZSM-5, with Si/Al
ratio 32) and silica were synthesized by incipient wet impregnation (Imp), in-situ (IS) and ion
– exchange (LSIE and SSIE) methods. The obtained catalysts were characterized by different
techniques (ICP, XRD, XRF, SEM, FTIR and nitrogen physisorption) and tested in
heterogeneous catalytic decomposition of ozone and oxidation reactions with ozone at
ambient temperature.
It has been observed a very high catalytic activity towards ozone decomposition for all
mesoporous catalysts and for some of microporous materials, while the catalysts remain
active with the time on stream. The conversion for the most active catalysts (5Ag(Imp)-H(IE)MCM-41-50, 5Ag(Imp)-SiO2, 5Ag(SSIE)-H(IE)-Beta-11 and 5Ag(Imp)-H(IE)-ZSM-5)
exceed 90% at room temperature. The activity of the catalysts enhances with increase of the
amount of supported silver, with decrease of the support acidity and modifying the catalyst
with some additional metals. The activity of a given catalyst can increase up to 7 times
depending on the method which is used for its synthesis.
The most active catalyst in the reaction of ozone decomposition - 5Ag(Imp)-H(IE)MCM-41-50, shows also high activity towards oxidation of CO and i-propanol with ozone. A
mechanism for ozone decomposition, based on the formation of two silver phases - AgO and
2
Ag2O(Ox) is proposed, which includes several reaction paths, without blocking the catalyst
surface.
Other possibility to achieve a high conversion at low temperature in reactions of
compete oxidation is to use an ordinary oxidant such as air or oxygen together with highly
active catalysts, containing nanosized gold supported on different transition metal oxides.
These catalysts are extraordinary active, especially in respect to CO oxidation, but are
unstable and their activity decreases with the time. That is why, the third object of our
research are two gold-supported catalysts and their behaviour during the catalytic tests.
The activity, long-term stability and the reasons for deactivation of nanosize goldsupported Au/TiO2 and Au/ZrO2 catalysts in CO oxidation were investigated. The catalysts
were prepared by a deposition-precipitation method and were characterized by chemical
analysis, adsorption measurements, XRD, XPS, FTIR, TEM, “depletive” oxidation and TPD
instrumental methods.
It was found that the nanosize gold-supported catalysts (Au/TiO2 and Au/ZrO2) contain
gold in both - metal and oxidized form. It has been observed a very high catalytic activity
towards CO oxidation for both of the gold catalysts even at temperatures below 0°C, as
Au/TiO2 catalyst is more active and has better long-term stability then Au/ZrO2 catalyst. In
spite of the experimentally proved very high activity, the two catalysts exhibited a gradual
decrease in initial activity with time.
It is proposed a model of reaction mechanism of CO oxidation on Au/TiO2 and Au/ZrO2
catalysts, which includes formation of an intermediate complex. This complex consists
connected both CO adsorbed on a gold particle as surface carbonyl and surface lattice oxygen
from the metal-support border. During CO oxidation it was observed formation and
accumulation of a monolayer of chemisorbed CO as surface carbonates. When this carbonate
layer stabilizes and covers all the surface of the catalyst, the access of new oxygen is blocked
and the gold particles are separated from the support, i.e. the formation of the active complex
is prevented. Therefore the reason for catalysts deactivation is the capability of gold catalyst
to adsorb CO and accumulate it as carbonates. This deactivation is reversible and the catalysts
liberate there surface by CO2 evolution after heating. During the catalytic operation, heating
and treatment, the catalysts additionally decrease there activity in result of destruction of the
fine structure and the high dispersion of the gold clusters caused by agglomeration. The
resulting deactivation is weak, but irreversible.
3